1. Field of the Invention
This invention relates broadly to orthopedic devices. More particularly, this invention relates to systems for engaging bone screws relative to bone plates.
2. State of the Art
For various fractures of bones of the body, compressive plating is a well known technique to impart the stabilization desirable for proper healing. In compressive plating, a rigid, typically metal plate is placed on the outer surface of the bone across the fracture, and screws extend through the plate and are secured into the bone on either side of the fracture in a manner which permits the rigid plate to offer support to the bone during healing. The screws include threads along a shaft adapted to engage cortical bone. Most commonly, the head portion of the screw is a standard screw head which provides a compressive force about a corresponding round screw hole of the plate as the fixator is threaded into the bone, thereby causing compression of the plate against the bone.
U.S. Pat. No. Re. 28,841 to Allgower describes a plate that is used with generally standard bone screws having heads with a convex undersurface. The plate includes oblong screw holes which each define at one end an upper ramped portion and a generally smaller radius of curvature about the ramped portion. In use, a hole is drilled into the bone through the screw hole adjacent the ramp and a screw is inserted into the drilled hole and rotated until the head of the screw contacts the ramp. Upon such engagement, there is displacement of the bone plate in a direction to move the ramped portion away from the screw and to cause the plate to apply pressure to maintain the bone parts together about the fracture in tight engagement. The holes in a such a plate are commonly referred to as dynamic compression holes (or DCH). However, micromotion between the bone and the portion of the screw within the bone can cause loosening of the entire assembly, diminishing the stability of the set fracture and a loss of compression across fracture.
More recently, threaded screws with heads which threadably engage in threads in the plate to lock the screws relative to the plate have been used. However, such systems do not provide the necessary control of compression between the plate and bone. Control over compressive forces is lost as soon as the threads of the head of the screw lock relative to the plate. Therefore, such a system provides sub-optimal stability for attachment of certain plates to bone. In addition, even such threaded engagement can loosen over time.
As a result, several systems have used secondary discrete locking elements to lock a bone screw to the plate. For example, U.S. Pat. No. 6,383,186 to Michelson teaches the use of a set screw which seats against the head of the bone screw to prevent backing out of the bone screw. U.S. Pat. No. 6,152,927 to Farris teaches a screw and washer assembly which provides compression against the head of the bone screw to lock the bone screw within the screw hole and prevent it from backing out. Both of these systems require that the surgeon work with separate and small locking elements at the time of the screw insertion, and such small elements may easily become lost in the surgical wound.
U.S. Pat. No. 5,549,612 to Yapp et al. teaches a system in which a screw can be locked relative to the plate with a cam permanently mounted in an aperture in the plate. The cam cannot provide any downward force against the screw head, thereby limiting potential fixation. As such, if the cam rotates just a small amount from a locking angle, the fixation provided by the cam may be lost. Moreover, the shape of the cam (as shown in FIGS. 4 and 4A of the patent) suggests that the cam applies an upward force against the screw head which disadvantageously counters the compressive force of the screw against the plate.